1. Field of the Invention
The present invention relates in general to recording mediums, such as an optical recording disc, an optical reproducing disc and the like and methods of producing the same. More particularly, the present invention relates to a method of molding a chucking hub member which is installed at a center portion of a disc substrate.
2. Description of the Prior Art
Hitherto, optical discs, such as a magneto-optical disc and the like are known as a recording medium on which various information signals can be recorded.
As one of drive devices for driving the optical discs, there has been widely used a type which employs a magnetic clamp system for achieving reduction in thickness of the device. The magnetic clamp system comprises generally a magnet mounted on a turn table of the disc drive device and a magnetic metal plate mounted to the optical disc. In use, the optical disc is put on the turn table and turned together with the same. Due to work of the magnetic force produced by the magnet, the optical disc is tightly attached to the turn table. The optical disc is constructed of a plastic, such as a polycarbonate or the like and provided at its center with a plastic chucking hub member which has the magnetic metal plate secured thereto. The plastic chucking hub member is welded to the disc by means of a ultrasonic welding.
One of the optical discs of this type is shown in U.S. Pat. No. 5,226,033, which will be described in the following to clarify the present invention.
FIGS. 1 to 4 of the accompanying drawings show the conventional optical disc.
In FIG. 1, denoted by numeral 1 is an optical disc which has both surfaces each being capable of recording and reproducing information signals. In fact, the disc 1 comprises two identical disc substrates 2 and 2 which are bonded at opposing surfaces thereof. The disc 1 is rotatably received in a cartridge (not shown).
Each disc substrate 2 is constructed of a plastic such as a polycarbonate or the like and has a center opening 2a. A chucking hub member 10 having a plastic portion is welded to the bored center portion 2a of the disc substrate 2 by means of a ultrasonic welding.
As is seen from FIG. 2, the chucking hub member 10 comprises an outer plastic ring 12, an inner plastic ring 13 and a circular magnetic metal plate 11 which is integrally molded with the outer and inner plastic rings 12 and 13. The metal plate 11 may be constructed of iron. The metal plate 11 is formed with a center opening 11a. A so-called "outsert molding method" is used for integrally molding the outer plastic ring 12 around the outer periphery of the metal plate 11. The outer plastic ring 12 has a plurality of portions which pass through openings 11b formed in the peripheral portion of the metal plate 11. With this molding method, the circular metal plate 11 is mounted on an outside annular surface of the outer plastic ring 12, as is understood from FIG. 2.
As is seen from FIG. 4 which shows the chucking hub member 10 turned over, the outer plastic ring 12 is formed at an inside annular surface with a circular ridge 12a and at its inner wall with a plurality of guide ribs 12b each having a head projected away from the metal plate 11 (see FIG. 2). The guide ribs 12b are equally spaced from one another. As is seen from FIG. 2, upon mounting the chucking hub member 10 to the disc substrate 2, the guide ribs 12b are brought into contact with a cylindrical wall of the center opening 2a of the disc substrate 2.
Similar to the above-mentioned outer plastic ring 12, the inner plastic ring 13 is integrally molded to the inner periphery of the metal plate 11 by means of an outsert molding method. The inner plastic ring 13 defines a centering opening 13a into which a spindle (not shown) of a disc drive device is inserted.
In fact, the outer and inner plastic rings 12 and 13 and the circular metal plate 11 are integrally molded by means of a known double outsert molding method which will be described hereinafter.
The chucking hub member 10 having the above-mentioned structure is secured to the disc substrate 2 in the following manner.
That is, first, as is seen from FIG. 2, the chucking hub member 10 is concentrically put on an operative surface of the disc substrate 2 having the guide ribs 12b mated with the cylindrical wall of the center opening 2a of the disc substrate 2. Thus, under this condition, a center-positioning of the chucking hub member 10 relative to the disc substrate 2 is established. Then, an ultrasonic generating horn (not shown) is put on the outer plastic ring 12 and energized for a predetermined time. With this, the circular ridge 12a formed on the outer plastic ring 12 is welded to the disc substrate 2.
Two identical disc substrates 2 and 2, each being constructed in the above-mentioned manner, are bonded at their opposing surfaces to constitute an optical disc 1.
Upon loading the optical disc 1, the metal plate 11 of the disc 1 is attracted by the magnet on the turn table of the disc drive device and the spindle of the drive device is inserted into the centering opening 13a of the disc 1 for centering or positioning the disc 1 relative to the turn table. Under this condition, the disc 1 is properly clamped by the magnetic clamp system.
As will be seen from FIG. 2, in molding the chucking hub member 10, injection gates for the outer plastic ring 12 are located at positions 12c (only one is shown) which face the outside surface of the chucking hub member 10, while an injection gate for the inner plastic ring 13 is located at a position 13c which faces the inside surface of the chucking hub member 10. That is, the injection gates for the outer and inner plastic rings 12 and 13 are positioned at opposite sides of the chucking hub member 10. As is seen from FIG. 3, upon molding, a melted plastic for the outer plastic ring 12 is injected into a cavity of a mold through the three injection gates (12c). Because of provision of the circular ridge 12a on the inside annular surface, the injection gates for the outer plastic ring 12 must be positioned at such outside position. That is, if the gates (12c) for the outer plastic ring 12 are positioned at the inside position like in the case of the inner plastic ring 13, three cuts would be required in the circular ridge 12a. Also, provision of such cuts in the ridge 12a lowers the bonding between the chucking hub member 10 and the disc substrate 2.
Accordingly, for production of the chucking hub member 10, a troublesome step for turning over the chucking hub member (more specifically, a semi-finish chucking hub member consisting of the metal plate 11 and the outer ring 12) has to be included in the series of production steps. This will be understood from the following description when taken in conjunction with FIGS. 5a and 5b.
That is, in order to produce the above-mentioned semi-finish chucking hub member, a mold assembly 20 as shown in FIG. 5a is used, which comprises a stationary mold 23 and a movable mold 21.
For the injection molding, the circular metal plate 11 is placed in a cavity 22 of the movable mold 21 and then the two molds 21 and 23 are tightly combined. Then, by operating an injection machine 24, melted plastic is injected through runners of the stationary mold 23 into the cavity 22 for molding the outer plastic ring 12. After the melted plastic is hardened, the movable mold 21 is dismantled from the stationary mold 23, and then the produced semi-finished chucking hub member is removed from the movable mold 21.
Then, another mold assembly 20' as shown in FIG. 5b is prepared, which comprises a stationary mold 23' and a movable mold 21'. The semi-finished chucking hub member is placed in a cavity 22' of the stationary mold 23' and the two molds 21' and 23' are tightly combined. Then, by operating another injection machine 26, melted plastic is injected into the cavity 22' for molding the inner plastic ring 13. After the melted plastic is hardened, the chucking hub member 10 thus consisting of the metal plate 11 and the outer and inner plastic rings 12 and 13 is removed from the mold assembly 20'.
As will be understood from the above description, in the conventional method, when the molding of the outer plastic ring 12 at the mold assembly 20 is finished, it becomes necessary to completely remove the semi-finished chucking hub member from the mold assembly 20 (more specifically, the movable mold 21) for preparation of the subsequent molding of the inner plastic ring 13 at the other mold assembly 20'. As is known, removal of the semi-finished chucking hub member from the mold assembly 20 involves troublesome and time-consumed manual labor. Furthermore, after removal from the mold assembly 20, the semi-finished chucking hub member must be turned over for proper setting in the other mold assembly 20', which makes the manual labor much harder.